A power supply can be configured to operate under a variety of power supply modes, e.g. the normal mode, which indicates that the output voltage of the power supply reaches the voltage level required by the specification; the standby mode, which indicates that the power supply is not connected to any load. In order to meet the requirements for safety regulation, the power supply is generally required to incorporate a power status indicator on the front panel thereof to display the current power status of the power supply. Generally speaking, the power status indicator of a power supply is implemented by a light-emitting device, such as a light-indicator consisted of light-emitting diodes, so that the current power status of the power supply can be displayed in the form of illumination during the operation phase of the power supply.
When the power supply is operating under the standby mode, that is, the power supply is operating under a no-load condition, the power supply will inevitably consume power by itself. The standby power consumption of the power supply generally represents the waste of energy. Nowadays energy saving and environmental protection are important issues to the contemporaries; the increase of power consumption means the increase of energy consumption and the deterioration of environmental pollution. Therefore, the reduction of the standby power consumption can efficiently improve the utilization of energy and reduce the emission of toxic gas. In recent years, some related departments and agencies of the United State and the European Commission have furthered the enhancement of energy efficiency and promulgated associated standards to reach the goal. For example, the department of energy (DOE) and the environmental protection agency (EPA) have jointly ratified an Energy Star Program. The program establishes the standards for a single-output power supply in 2005 as: for a power supply with an output power of 0-10 watts and operating under a no-load condition, the maximum input power can not exceed 0.5 watt; for a power supply with an output power of 10-250 watts and operating under a no-load condition, the maximum input power can not exceed 0.75 watt. Such standards would become stricter in the near future.
FIG. 1 is a circuit block diagram showing a power supply according to the prior art. As shown in FIG. 1, a power supply 100 includes an AC-DC converter stage 101 for converting an input AC voltage Vin into a desired output DC voltage Vo so as to power a load (not shown), and a pulse-width modulation controller (PWM controller) 102 connected to the AC-DC converter stage 101 for controlling the switching operations of the switches located within the AC-DC converter stage 101, so that the output voltage of the AC-DC converter stage 101 can be regulated at a predetermined level. The power supply 100 further includes a feedback circuit 103 connected between the output terminal of the AC-DC converter stage 101 and the PWM controller 102 for detecting the variation on the output voltage of the AC-DC converter stage 101 and sending a feedback control signal to the PWM controller 102 based on the detection results so as to accomplish the regulation for the output voltage. The power supply 100 further includes a protection circuit 104 connected between the output terminal of the AC-DC converter stage 101 and the PWM controller 102 for shutting down the PWM controller 102 in case of abnormality encountered by the output voltage or the output current of the AC-DC converter stage 101, so that the AC-DC converter stage 101 can be protected from being damaged by over-voltage or over-current problems.
The power supply 100 of FIG. 1 also includes a power status indicator 105 connected across the voltage output terminals of the power supply 100. The power status indicator 105 includes a light-emitting diode D and a current-limiting resistor R being connected in series with the light-emitting diode D, as shown in FIG. 1. Generally, the voltage drop of the light-emitting diode D is about 1-2 volts, and the driving current of the light-emitting diode D is as low as several milli-amperes. When the output voltage of the power supply 100 is high, for example, about 10-20 volts, the majority of the power consumption of the power status indicator 105 are contributed by the current-limiting resistor R. When the power supply 100 is operating under the standby mode, the power consumption of the power status indicator 105 represents a non-negligible power loss. Therefore, the current-limiting resistor R is a major contributor to the standby power consumption. It is therefore an important task to achieve a energy-saving power supply by reducing the standby power consumption caused by the current-limiting resistor R. The present invention can address these problems.